A wide variety of ingredients have been suggested for use in laundering operations to enhance the appearance of fabrics. Detergents, of course, provide a basic cleaning function. Rinse-added fabric softeners provide both softening and anti-static benefits to fabrics. More recently, cellulase enzymes have been employed to improve the appearance of colored cotton garments.
Formulators of fabric cleaning products have clearly recognized the need to improve the color fidelity of dyed fabrics. As noted above, the use of cellulase is one modern method for achieving this desirable result. Other formulators have approached this challenge from the standpoint of more effective cleaning. For example, various bleaches are advertised as being able to maintain color brightness. Another means for addressing the problem of color fidelity employs dye transfer inhibiting agents in the laundering liquor. This approach is based on the discovery that vagrant dyes present in the laundering liquor can undesirably redeposit onto fabrics, thereby gradually changing, and generally darkening, colors and whites. While the use of cellulases, dye transfer inhibiting agents and bleaches can meet certain consumer needs for maintaining color fidelity, there is a continuing search for improvements in this area.
The present invention addresses the problem of color fidelity in laundered fabrics from an entirely different aspect. It has now been determined that metal cations, especially transition metals, and most particularly copper and nickel ions, present in aqueous rinse baths can undesirably interact with fabric dyes and change their perceived hue. This also often translates into a darkening of the dye material, which tends to cause the colored fabrics to appear drab. Interactions of metal ions with residual soils may also tend to clear a drab appearance. While many conventional washing compositions contain metal ion sequestrants which may minimize this problem during the actual washing operation, it has heretofore been overlooked that the freshly laundered fabrics are subsequently subjected to aqueous rinse baths which do not contain such sequestrants. It has now been discovered that metal ions present in the rinse can also undesirably interact with dyed fabrics, resulting in a loss of color fidelity and brightness.
While not intending to be limited by theory, it may be speculated that functional substituent groups present in complex dye molecules bind with metal ions, thereby causing a change in color which is generally perceived as drabness and an overall appearance of fabric aging. This has now been found to occur with common ortho-hydroxy diazo dyes and with certain direct dyes. A similar undesirable interaction may also occur between metal cations and the "optical brighteners" which are commonly used to enhance the perception of whiteness and brightness of white fabrics, thereby resulting in reduced fluorescence of the fabrics. Whatever the reason for the drabness and change in appearance, it has now been discovered that such problems associated with loss of color fidelity can be overcome by the fabric treatment process herein which is conducted in the rinse bath.
By the practice of the present invention, dyed or white fabrics are rinsed in an aqueous rinse bath which contains a metal ion chelating agent. The chelating agent is present in an amount sufficient to scavenge metal ions, especially copper and nickel, thereby preventing undesirable metal interactions with dyes or optical brighteners. Moreover, the invention also can be used to remove metal ions which have already combined with dye or optical brightener molecules on fabrics in the laundering process, thereby providing a restorative benefit to colors which have become drab due to metal ion interactions, especially due to interactions with copper cations and nickel cations, but also manganese cations, iron cations, and transition metal cations, among others. These and other objects are secured by the present invention, as will be seen from the following disclosure.